Phosphorus (P) is an essential element for plant growth and development. Vacuoles play a fundamental role in the storage and remobilization of P in plants, while our understanding of the evolutionary mechanisms of creating and reusing P stores are limited. Besides, we also know very little about the coordination of intercellular P translocation, neither the inorganic phosphate (Pi) signaling nor the Pi transport patterns. Here we summarize recent advances in understanding the core elements involved in cellular and/or subcellular P homeostasis and signaling in unicellular green algae and multicellular land plants. We also propose further work that might help to uncover the high-resolution intra- and inter-cellular landscape of Pi distribution and signaling in plants.
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Schematic summary of core elements involved in phosphate transport and signaling in unicellular green algae and multicellular land plants.
(a) In unicellular green algae, taking Chlamydomonas as an example, phosphorus is stored in vacuole as polyphosphate (polyP). SPX-VTCs (containing both SPX and VTC domain, SPX named after SYG1/Pho81/XPR1, VTC named after vacuolar transporter chaperone) and SPX-SLCs (containing both SPX and SLC domain, SLC named after permease solute carrier 13) proteins take charge of influx/synthesis and efflux transport of vacuolar phosphorus, respectively. Upon inorganic phosphate (Pi) starvation, PSR1 expression is induced, which further induces the expression of phosphate starvation-responsive (PSR) genes. PTAs (named after Pi Transporter A) and PTBs ((named after Pi Transporter B)) are indicated to mediate the Pi uptake across the plasma membrane (PM), while the Pi transporter(s) located on the PM involving in Pi efflux is still unknown.
(b) In multicellular land plants, phosphorus is stored as Pi in the central vacuole. Under Pi-sufficient condition (left panel), cellular ATP levels are increased, leading to increased VIHs (named after VIP1 homologs) kinase activity and subsequent increase of InsP8 content. Then InsP8 accumulates and triggers the formation of an SPX–InsP8–PHR (PHR named after Phosphate Starvation Response) complex, which blocks the activity of PHRs. Tonoplast-located SPX-MFS (containing both SPX and MFS domain, MFS named after Major Facilitator Superfamily) and VPE (named after vacuolar Pi efflux transporter) proteins act as vacuolar Pi influx and efflux transporters, respectively. Under Pi-deficient condition (right panel), cellular ATP levels are decreased, and consequently, InsP8 content is reduced. Then PHRs are released in the nucleus, which further activates the expression of PSR genes. Intercellular Pi transports are mediated by symplastic and apoplastic pathways. The symplastic pathway could be divided into two routes based on Pi transport by the endoplasmic reticulum (ER) and/or interspaces in plasmodesmata (PD). Arrows on the transporters show the transporting directions, and thick arrows indicate induced transport activity. SPX domains of SPX-containing proteins are visualized as blue-filled circles. The unknown transporters are marked with grey symbols, and putative transport or regulation pathways are marked with dashed lines. CW, cell wall. PM, plasma membrane.